Ukraine's acting President Oleksandr Turchynov has called for the relaunch of an "antiterrorist" operation against pro-Russian rebels in eastern Ukraine, citing the death of a politician from his party in the region.

Turchynov said in a statement that two "brutally tortured" bodies were found near the rebel-held city of Slovyansk on April 22.

One was that of Volodymyr Rybak, a member of Turchynov's Batkivshchyna (Fatherland) party. The president said Rybak had recently been abducted by "terrorists."

Turchynov said, "These crimes are being carried out with the full support and indulgence" of Russia.

In his statement, Turchynov said, "I call on the security agencies to relaunch and carry out effective antiterrorist measures, with the aim of protecting Ukrainian citizens in the east from terrorism."

Donetsk regional police said the body of a man who died a violent death had been found and that it resembled that of Rybak, a local councilor in the town of Horlivka, near Donetsk. Police said formal identification would require further work.

Ukrainian media said Rybak was abducted in Horlivka last week.

Turchynov's announcement marks the second time that an "antiterrorist" operation has been launched in the east.

Ukraine's security forces announced an "antiterrorist" operation in the region on April 15. But it appeared to have limited effectiveness, and pro-Russian separatists remained in control of around 10 towns in the mainly Russian-speaking east.

Authorities officially suspended the military operation for last weekend's Easter holiday, in what was described as a goodwill gesture.

Ukraine, Russia, the United States and the European Union on April 17 signed an agreement in Geneva to defuse tensions in eastern Ukraine. The deal calls for illegal armed groups to surrender their weapons and leave public buildings they occupy.

Pro-Moscow insurgents holding Slovyansk and other eastern towns have ignored the Geneva agreement. Ukraine and Russia have accused each other of failing to implement it.

Journalist Held

The self-declared separatist mayor of Slovyansk, Vyacheslav Ponomaryov, told a press conference on April 22 that he was "preparing to repel an attack" because the city "might be stormed even tonight."

He also confirmed that his men were holding a journalist.

"A journalist has been detained as far as I understand," Ponomaryov said. "To be honest I didn't have time this morning to solve this problem and meet him to talk to him."

Ponomaryov did not name him but journalists working in the area said Simon Ostrovsky had been detained by gunmen on April 21 and, unlike others taken with him, had not yet been released.

Vice News said on its website it was "aware of the situation and is in contact with the U.S. State Department and other appropriate government authorities to secure the safety and security of our friend and colleague, Simon Ostrovsky."

Ostrovsky has been covering the crisis in Ukraine for weeks. His last posting on Twitter was on April 21, when he covered an earlier press conference by Ponomaryov.

He noted that during the press conference a woman working with the separatists "called us all liars," and then asked journalists for donations for the funerals of militants who had been killed in a gunfight on April 20.

Ukrainian Plane Shot At

In another development, officials said a Ukrainian military surveillance plane came under small-arms fire on April 22 as it flew over Slovyansk. The Defense Ministry said the plane was able to make a safe landing, and no injuries were reported.